1. Field of the Invention
This invention generally relates to systems and methods for inspection of a specimen. Certain embodiments relate to an inspection system that includes a non-imaging detection subsystem configured to generate output signals responsive to light specularly reflected from a spot scanned across the specimen and a processor configured to generate images of the specimen using the output signals.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. When inspecting specular or quasi-specular surfaces such as semiconductor wafers, bright field (BF) and dark field (DF) modalities are used. In BF inspection systems, collection optics are positioned such that the collection optics capture a substantial portion of the light specularly reflected by the surface under inspection. In contrast, in DF inspection systems, the collection optics are positioned out of the path of the specularly reflected light such that the collection optics capture light scattered by objects on the surface being inspected such as microcircuit patterns or contaminants on the surfaces of wafers.
In BF inspection systems such as the 2351 system that is commercially available from KLA-Tencor, San Jose, Calif., imaging optics are commonly used to direct the light specularly reflected from the surface being inspected to the surface of an imaging sensor (e.g., an array detector such as a charged coupled device (CCD) or photodiode array). The quality of the imaging optics is a crucial determinant of the image quality overall, and the imaging optics (e.g., the numerical aperture of the imaging optics) need to be carefully matched to the periodicity of the imaging detector (e.g., the periodicity of the detector pixels) to prevent sampling effects such as aliasing.
In DF inspection systems such as the AIT family of tools and the SP1 and SP2 tools that are commercially available from KLA-Tencor, an intense spot of light is commonly used (e.g., light generated by a monochromatic laser) to limit the extent (i.e., the area) of the surface being illuminated. The inspection systems are configured to collect scattered light from the surface under inspection using non-imaging optics (such as Fresnel lenses or curved mirrors) and to direct the collected light onto the surface of relatively large area sensors (such as photodiodes or photomultiplier tubes). The inspection systems are also configured to sequentially illuminate different regions of the surface under inspection by scanning the spot over the surface by either translating the surface under the optics of the inspection system or by steering the illumination beam using devices such as galvanometers, rotating polygonal mirrors, or acousto-optic deflectors. The DF inspection systems are configured to form a digital image by using knowledge of the position being illuminated at the time the sensor is sampled. Since the non-imaging or “acquisition” optics collect a substantially small portion of the light used for illumination, relatively intense light sources such as lasers are typically used in DF inspection systems to illuminate the spot on the surface under inspection such that sufficient photons can be collected to provide a sufficient signal-to-noise ratio for defect detection.
Some DF inspection systems are configured to scan multiple spots across the surface under inspection simultaneously. In such systems, a relatively sparse array of detectors can be used to increase the speed of data acquisition by parallelization. Such a system configuration requires somewhat more sophisticated imaging optics than single spot DF inspection systems, but the tolerances of the optics can be substantially relaxed from those used in bright field systems.
Accordingly, it would be advantageous to develop systems and methods for BF inspection of a specimen such as a wafer that can produce adequate quality images of the specimen without requiring high quality imaging optics, without matching the optics to the periodicity of the detector, and without causing sampling effects such as aliasing, that allows the use of optics having a relatively high numerical aperture to increase the efficiency of light collection, that allows the use of detectors having a relatively high gain, that is less expensive than other bright field inspection systems, or some combination thereof.